Diagnosing atmospheric communication of a sealed monitor chamber.

Journal Article (Journal Article)

Daily output variations of up to ±2% were observed for a protracted time on a Varian TrueBeam® STx; these output variations were hypothesized to be the result of atmospheric communication of the sealed monitor chamber. Daily changes in output relative to baseline, measured with an ionization chamber array (DQA3) and the amorphous silicon flat panel detector (IDU) on the TrueBeam®, were compared with daily temperature-pressure corrections (PTP ) determined from sensors within the DQA3. Output measurements were performed using a Farmer® ionization chamber over a 5-hour period, during which there was controlled variation in the monitor chamber temperature. The root mean square difference between percentage output change from baseline measured with the DQA3 and IDU was 0.50% over all measurements. Over a 7-month retrospective review of daily changes in output and PTP , weak correlation (R2  = 0.30) was observed between output and PTP for the first 5 months; for the final 2 months, daily output changes were linearly correlated with changes in PTP , with a slope of 0.84 (R2  = 0.89). Ionization measurements corrected for ambient temperature and pressure during controlled heating and cooling of the monitor chamber differed from expected values for a sealed monitor chamber by up to 4.6%, but were consistent with expectation for an air-communicating monitor chamber within uncertainty (1.3%, k = 2). Following replacement of the depressurized monitor chamber, there has been no correlation between daily percentage change in output and PTP (R2  = 0.09). The utility of control charts is demonstrated for earlier identification of changes in the sensitivity of a sealed monitor chamber.

Full Text

Duke Authors

Cited Authors

  • McCaw, TJ; Barraclough, BA; Belanger, M; Besemer, A; Dunkerley, DAP; Labby, ZE

Published Date

  • August 2020

Published In

Volume / Issue

  • 21 / 8

Start / End Page

  • 309 - 314

PubMed ID

  • 32648368

Pubmed Central ID

  • PMC7484838

Electronic International Standard Serial Number (EISSN)

  • 1526-9914

Digital Object Identifier (DOI)

  • 10.1002/acm2.12975

Language

  • eng

Conference Location

  • United States